1. The Field of the Art.
The present invention relates to a cathode structure for an electron gun used in a cathode ray tube, and particularly to a cathode structure in which the cathode is fixedly coupled with a cup shaped control grid through an insulator.
2. Background of the Invention.
Generally, industrial cathode ray tubes such as computer monitors require a high level of mechanical stability and precision. One such known cathode structure 10 is shown in FIGS. 1 and 2, which includes three cathodes 12, 14, 16 as the sources of three electron beams, the cathodes being installed within a center open portion of an insulating block 22 made of glass or ceramic. This assembly is then disposed within a cup shaped control grid 20 made of conductive material, typically stainless steel. As illustrated, the cup shaped control grid contains a top portion 18A and side portions 18B, C, D and E.
The three cathodes 12, 14 and 16 emit electrons and are fixedly aligned in a line along a longitudinal axis of an insulating block 22 having two ridges 24 and 26 formed along the longitudinal edges thereof. The insulating block 22 is then inserted into the cup shaped control grid 20 so that the control grid 20 is electrically isolated from cathodes 12, 14 and 16.
A supporting piece 28 made of metal is provided on the bottom of the insulating block 22 to prevent detachment of the control grid 20, and the supporting piece 28 is welded to the inside of the skirt of the control grid 20. Coupling portions 30 of supporting piece 28 are formed at the opposite ends of the supporting piece to fit into fitting grooves 32 formed at the bottom of insulating block 22.
With this conventional cathode structure, after the cathode carrying insulating block 22 is inserted into the cup shaped control grid 20 and the supporting piece 28 is welded to control grid 20, the supporting piece 28 prevents detachment of the insulating block 22 and desirably keeps the ends of each cathode 12, 14 and 16 a desired gap G on the order of, for example, 0.1 to 0.2 mm, shown in FIG. 2, from the beam passing holes 34, 36 and 38, respectively, of the control grid 20. However, with this cathode structure, there is the possibility that slight movements of the insulating block 22 can occur in tiny gaps (not shown) between the insulating block 22 and control grid 20 because only the supporting piece 28 secures insulating block 22 within the control grid 20.
Particularly, a heater (no shown) provided within the cathodes 12, 14 and 16 causes the control grid 20, having a higher thermal expansion coefficient relative to the insulating block 22, to expand more and cause a larger gap between the control grid 20 and the insulating block 22. Due to this larger gap, undesired movement of the insulating block 28 is increased.
Such an increased movement of the insulating block 22 becomes visibly obvious when an external impact is applied and the relative movement increases of the cathodes 12, 14 and 16 with respect to the beam passing holes 34, 36 and 38 of the control grid 20, with the result that the picture is garbled or vibrated.
It should also be noted that with this cathode structure, projected portions 30 maintain the gap G between each cathode and beam passing hole. However, it is extremely difficult to repeatedly obtain the high precision dimensional conditions required for high precision projection. One such specific instance is when glass or ceramic is used as the principal insulating material and a high precision projected portion cannot be expected.
Therefore, it is apparent that a cathode ray tube having a high level stability and a high precision cannot be expected from the conventional cathode. Further, an extra portion, for welding the supporting piece which is a means for securing the insulating block has to be provided on the skirt portion of the control grid, and therefore, the control grid becomes longer in its length. This ultimately brings the result that the electron gun or the cathode ray tube is elongated, thereby making it impossible to miniaturize it.